The Powerglide transmission was one of the first automatic transmissions developed by General Motors. Although General Motors phased out the Powerglide transmission in 1973, the basic design is still used today, especially in niche automotive applications, including in automobile power trains designed for racing. The Powerglide transmission has remained popular for racing due, in part, to the strength, durability, and simplicity of the design. Indicative of the Powerglide's continued popularity, it is today possible to build an entire Powerglide transmission from aftermarket parts, and a cottage industry has developed around improving the performance of the Powerglide transmission in racing applications. Because of this history, entire transmissions as well as complete replacement parts built to original equipment (“OE”) dimensions and specifications continue to be readily available in the market. Thus, for the sake of clarity, “OE Powerglide” as used herein refers not only to Powerglide transmissions and parts originally manufactured by General Motors but also to any aftermarket transmissions or parts conforming to the OE dimensions and specifications.
Because the present invention relates to new and nonobvious modifications of the OE Powerglide design, it is helpful to have a basic understanding of the OE Powerglide transmission to assist in understanding embodiments of the present invention. For this reason, FIGS. 6 and 7 are included to illustrate the OE Powerglide configuration. As shown in FIG. 6, OE Powerglide transmission 100 includes a clutch drum 110 that is rotatably coupled to a planetary gear portion 112. Power can be transmitted from, for example, an automobile engine (not shown) through an input shaft 114, the clutch drum 110, and the planetary gear portion 112, to an output shaft 116. The planetary gear portion 112 includes a two-speed compound planetary gear set coupled to a reverse clutch 130. The compound planetary gear set can include an input sun gear 118, a reaction sun gear 120, three long pinions 124 and three short pinions 126 and a ring gear 128 (only one long pinion 124 and one short pinion 126 are illustrated). As shown in FIGS. 6 and 7, the OE Powerglide input sun gear 118, reaction sun gear 120, long pinions 124, and short pinions 126 each have a substantially constant maximum outer diameter. Maximum outer diameter, as used herein, refers to the diameter of the gear measured from the tops of the gear teeth. The long pinions 124 and short pinions 126 are movably coupled to a planet carrier 122 which is coupled to the output shaft 116. As shown in FIGS. 6 and 7, the input sun gear 118 is meshed to the long pinions 124, which are meshed to the short pinions 126, which are in turn meshed to the reaction sun gear 120 and planetary ring gear 128. The OE Powerglide transmission 100 can be operated in different modes depending on the configuration of the various clutches and bands. For example, in first gear, a low band 132, which is grounded to the transmission case, is applied, which holds the clutch drum 110 and reaction sun gear 120 stationary. The input shaft 114 is coupled to the input sun gear 118 so that, when the input shaft 114 turns, for example in the direction indicated by arrow A, the input sun gear 118 also turns in that direction, as indicated by arrow B. Turning the input sun gear 118 causes the long pinions 124 to rotate, which causes the short pinions 126 to rotate. Because, in first gear, the clutch drum 110 and reaction sun gear 120 are held stationary by the low band 132, rotation of the input sun gear 118 forces the short pinions 126 to “walk” around the reaction sun gear 120 and turn the planetary carrier 122. Because the planetary carrier 122 is coupled to the output shaft 116, this also turns the output shaft 116.
FIG. 7 illustrates a cross-sectional view of the planetary gear portion 112 of the OE Powerglide transmission 100. As shown in FIG. 7, the reverse clutch 130 can include one or more fiction discs, or frictions, 134 that are coupled to the ring gear 128. The reverse clutch 130 also includes one or more separators 136 that are fixed to an inner surface 400 of a transmission case. The ring gear 128 can include a proximal gear portion 150 that meshes with the short pinions 126 and a distal flange portion 152 that couples to the reverse clutch 130 via the frictions 134. In operation, the reverse clutch 130 can be applied to hold the ring gear 128 stationary by applying an axial force in the direction indicated by arrow C with piston 138. Applying an axial force with the piston 138 can cause the friction discs 134 to be squeezed between the separators 136, which are grounded to the transmission case, holding the friction discs 134 and thus the ring gear 128 stationary.
The Powerglide transmission, in both OE designs and modern racing modifications, is a two-speed automatic transmission that includes a compound planetary gear assembly. OE Powerglide transmissions are sold with two different numerical first gear ratios. In one, the first gear, also known as the “low gear,” has a numerical ratio of 1.76:1 and the second gear has numerical ratio of 1:1. This means that, in first gear, the input shaft must turn 1.76 rotations for one full rotation of the output shaft. The other OE Powerglide model has a numerical first gear ratio of 1.82:1 and a numerical second gear ratio of 1:1.
In applications with high horsepower to vehicle weight ratios, such as drag racing, it can be desirable to have a lower numerical first gear ratio to improve tire traction. For example, a lower numerical first gear ratio can improve tire traction when the vehicle launches from a stationary position by reducing the torque applied to the wheels. Various attempts have been made to reduce the numerical first gear ratio of the Powerglide transmission below the manufacturer-provided ratio of 1.76:1. Current designs, however, have failed to reduce the ratio below 1.65:1.
The first gear ratio of the Powerglide transmission can be calculated with the following equation:
      Numerical    ⁢                  ⁢    First    ⁢                  ⁢    Gear    ⁢                  ⁢    Ratio    =      1    +                  Number        ⁢                                  ⁢        of        ⁢                                  ⁢        Teeth        ⁢                                  ⁢        on        ⁢                                  ⁢        Reaction        ⁢                                  ⁢        Sun        ⁢                                  ⁢        Gear                    Number        ⁢                                  ⁢        of        ⁢                                  ⁢        Teeth        ⁢                                  ⁢        on        ⁢                                  ⁢        Input        ⁢                                  ⁢        Sun        ⁢                                  ⁢        Gear            Accordingly, to decrease the numerical first gear ratio, the number of teeth on the input sun gear must be increased relative to the number of teeth on the reaction sun gear. However, space constraints within the Powerglide transmission case limit the extent to which the size of the input sun gear can be increased, and thus, limit the extent to which the numerical first gear ratio can be reduced. Some modified designs have attempted to accommodate a larger input sun gear by using a “stepped down” long pinion, which has two different portions, with each portion having a different number of teeth. However, this “stepped down” design is weaker and therefore undesirable.
Another aspect of the Powerglide transmission that has been the focus of modifications is the reverse clutch. Often when a Powerglide transmission is used for drag racing, it is modified so that the first gear and reverse gear can be engaged simultaneously. The first gear is engaged by applying the low forward band and the reverse gear is engaged by applying the reverse clutch. Engaging the first gear and reverse gear simultaneously allows the transmission to be used as a brake, holding the vehicle stationary with the engine revving to a higher RPM through the fluid coupling of the torque converter. The vehicle can then launch forward immediately when the reverse clutch is released. However, to effectively hold the vehicle stationary at near-maximum engine power, the holding capacity of low forward band and reverse clutch must be maximized. One way to increase the holding capacity of the low forward band and reverse clutch is to increase oil line pressure, however, this robs horsepower from the drivetrain system. While there are several alternative options available for increasing the holding capacity of the low forward band, there are minimal alternatives for increasing the holding the capacity of the reverse clutch.
Accordingly, there is a need for a lower numerical first gear ratio in a Powerglide-based transmission design. There is also a need for a high torque capacity reverse clutch that does not rob power from the drivetrain system.